The dual quality of light, “that it is both particle and wavelike”, brought a revolution to modern scientific thinking. Lasers, which are a a kind of light and an outgrowth of quantum mechanics, have essential roles in many phases of everyday life in the recent years. Also in the clinical routine of dentistry, lasers are beginning to see more use. Industry has been successful in making the delivery apparatus much smaller and advancing the fibre optic technology and more importantly, achieving better cost performance.There have been numerous studies to date on laser ever since the invention by Dr Mainman in 1960. As is well known, it was begun with ablation of a tooth by laser, which lead to discovery of its improved acid resistant quality. Ruby lasers were used in the early stage, but due to their effect on dental pulp, interest was shifted to the studies with other kinds of laser. Clinical applications require lasers to be used in extremely confined spaces such as pits, fissures and root canals, for which light guide systems and tips were developed. In today's clinical dentistry, dental applications by a wide variety are being studied.The lasers used for clinical applications are mainly CO2, Nd: YAG, Er: YAG, Ar, and Diode. Among the latest models of laser device recently adopted for clinical applications, there are those using such mediums as Er, Cr: YSGG and Nd: YAP.Studies are beginning to be made for understanding reactions of various lasers with different materials using a free electron laser apparatus.Significance of the studies is based on the fact that, depending on each different wavelength, absorption and reflection of laser light on the surface of materials vary. And absorption level of energy varies under the surface according to composition, colour, water content etc. of the materials. Irradiation by laser on different parts of human body will obtain various reactions depending on wavelength, output cacpcity tissue, laser medium, etc.Applications of laser are made in the extensive fields of prevention, prosthesis, oral surgery, orthodontics etc. (Table 3). Nevertheless, there is a significant variety of problems that need to be resolved before lasers become really safe and effective to use in dentistry.In this sense, now is the time when the following questions require answers with proper explanations and detailed demonstrations by both researchers and clinicians: “Are lasers really effective?”;“What are the conditions in order to achieve real effectiveness?”